Hi B,
I just came back from your lovely UCSD campus, I hope it's not being
overrun by summer students.
It would be a lot easier to make a decisive comment on this if I could
see a waveform.
My first suggestion is to move from recording fEPSPs to looking at
population spikes. Because the reflected EPSP goes in the other
direction to to fiber volley, you should have to trouble telling it
from the any other parts of the waveform.
You stimulus artifact shouldn't be invading your fiber spike. If it
is, I would look at the placement of the electrodes, the stimulating
electrode or the stimulus isolator. If your electrodes are very close
together, then the conduction lag wont be enough to seperate the fibre
spike from the stimulus artifact. Furthermore, you are more likely to
get capacitive coupling between the electrodes. Hopefully, you're
stimulus artifact is mainly monopolar, i.e. it just goes up the page,
or down the page, but not both. You should be able to flip the
polarity of your stimulus, which should help seperate it from the
fiber spike.
What kind of stimulating electrode are you using? Monopolar, bipolar,
two big fat wires? Theta glass? In my experience, you get the smallest
artifact (amp for amp) from a monopolar glass pipette electrode, with
the ground somwhere away from your recording site. But of course the
resistance is going to go up relative to a wire electrode, so you
might not be able to deliver enough current to do the business.
The worst artifact I ever saw was when a guy connected the earth from
his stimulator to the earth for his recording (exponentially decaying
wave with a time constant of 20ms). You need to make sure you're
stimulus isolator is still working, and that you're not laying the
cable that caries your stimulus pulse over the cables that carry
signal.
But these are all just general ideas. How broad is your stimulus
artifact, and how broad is your stimulus waveform (i.e. whats coming
out of the box)?
However, you need to remember, fibre spikes are often vaguely hard to
see in fEPSPs, and usually invisible if you're above room temperature.
If you want them nice and clear, you're going to have to block
glutamatergic synaptic traffic (do a google images search for fEPSP if
you don't believe me).
Hope I've been some help.
On Jul 1, 4:29 pm, bmidt... from ucsd.edu wrote:
> Hi,
>> I am new to the electrophysiology world and am performing hippocampal
> field recordings on acute slices. I am currently getting a nice field
> response, however, many times I barely see a fiber volley, (if at all)
> which will sometimes decrease as I increase stimulation. Additionally,
> because they (the fiber volleys) are so small, any tiny variation
> radically changes the input/output slopes. My feeling is that this is
> due to the large stimulation artifact. Does anybody have any ideas on
> what may be causing this and how to fix it?
>> Thanks,
> B~